We are studying the relation between recognition of "low-efficiency" markers in homospecific DNA and discrimination against heterospecific DNA. To this end we are seeking to isolate mutants defective in the ability either to recognize "low-efficiency" markers or to discriminate against heterospecific DNA. Mutants of the one type are then checked to determine whether the other capacity is intact. We are also examining the fate of heterospecific DNA in discriminating recipients by following its transforming activity in extracts obtained from recipients lysed at various times after uptake. These extracts are tested on both recipients of the same species as that of the original recipient as well as recipients of the donor species.